1. Technical Field
The present invention relates to a gas analyzer using a surface acoustic wave device, and a method of gas analysis.
2. Background Art
Chemical substances in the environment have serious effects on human beings, and there are many kinds of unpredictable hazardous and toxic gases present in living spaces, production sites, transportation facilities, and other areas. Hence, there is a demand for a sensing technique of immediately measuring such gases by a portable compact sensor and setting off alarm. In most cases, no prior information on the kinds of gases present can be obtained. It is therefore necessary to separate and detect more than 100 kinds of various hazardous and toxic gases; gas sensors for detection of only several kinds of specific gases are insufficient. Further, since a gas sensor generally responds to a plurality of gases, a wide variety of gases can hardly be detected even by as many sensors as the number of gases. For this reason, methods using pattern recognition or multivariate analysis of response patterns of several sensors having different gas responses have been proposed. However, when several gases reach the sensors at the same time, such methods using pattern recognition problematically fail to detect the gases.
As stated above, the measurement of simultaneously reaching several gases is difficult for portable sensors, but this measurement is possible by analyzers installed on the floor or table. In particular, a gas chromatograph is a typical example of such analyzers. A gas chromatograph makes use of a phenomenon that when a plurality of gases pass through the inside of a column, e.g., a packed column filled with liquid-coated particles and a capillary column to the inside of which a liquid is applied, the difference in solubility between the gases and the liquid makes a difference in the pass times of the gases; and the gas molecules are detected by a detector disposed at the outlet of the column. A wide variety of gases can be identified and measured by one or a few sensors.
For example, a flat surface acoustic wave (SAW) sensor (see, e.g., Patent Literature 1) is used as a detector for a gas chromatograph. FIG. 7(a) shows a surface acoustic wave sensor 50 comprising an oscillator circuit, in which surface acoustic waves propagate on a base material 51, and then is fed back to an input terminal as electric signals. This sensor is compact in size and easy to arrange in an arrayed shape. In the surface acoustic wave sensor 50, mass loading and elastic loading on a sensitive film 52 due to the influence of gas molecules are measured as changes in frequency (or velocity) of surface acoustic wave.
Meanwhile, the present inventors found that surface acoustic waves traveled along the surface of a ball many times (over 100 times) more than expected. Analyzing the cause, they discovered nondiffracting beams (see Non Patent Literature 1). This is a phenomenon in which surface acoustic waves on a ball are influenced by two effects, i.e., diffraction, as a universal phenomenon of waves, and focusing by the geometric feature of a ball, and as a result of the balance of them, a narrowly collimated (parallel) beam is naturally formed. Thus, the surface acoustic wave is not influenced by obstacles, strains, defects, etc., of portions other than the narrow beam. Therefore, the wavefront is precisely maintained and attenuation is low, permitting multiple rounds of the surface acoustic wave. The inventors also revealed that the conditions for forming non-diffraction beams are such that the width of the beam is equal to the geometric mean of the diameter of a ball and the wavelength of a surface acoustic wave. Here, the influence of diffraction disappears, which is a phenomenon beyond common sense in physics. This is a new fundamental discovery made after over 100 years since the discovery of surface waves by Rayleigh in 1885, and this discovery is useful for great improvement in the sensitivity of the surface acoustic wave device. More specifically, in a spherical surface acoustic wave sensor 60 utilizing this phenomenon, as shown in FIG. 7(b), since a surface acoustic wave propagates for a long distance by multiple rounds, attenuation changes are amplified in proportion to the number of turns, and are thus measurable as amplitude variations (see Non-Patent Literatures 1 through 3). Surface acoustic waves were known to travel along the surface of a cylindroid, cylinder, cone, ball, and the like (see, e.g., Patent Literature 2); however, the above-described facts that surface acoustic waves travel along the surface of a ball, free of influence of diffraction within a band having a predetermined width, and conditions thereof were first found by the present inventors, and have been put to practical use.